PIPE(2)                                       Linux Programmer's Manual                                       PIPE(2)

NAME
       pipe, pipe2 - create pipe

SYNOPSIS
       #include <unistd.h>

       /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64; see NOTES */
       struct fd_pair {
           long fd[2];
       };
       struct fd_pair pipe();

       /* On all other architectures */
       int pipe(int pipefd[2]);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <fcntl.h>              /* Obtain O_* constant definitions */
       #include <unistd.h>

       int pipe2(int pipefd[2], int flags);

DESCRIPTION
       pipe() creates a pipe, a unidirectional data channel that can be used for interprocess communication. The ar‐
       ray pipefd is used to return two file descriptors referring to the ends of the pipe. pipefd[0] refers to the
       read end of the pipe. pipefd[1] refers to the write end of the pipe. Data written to the write end of the
       pipe is buffered by the kernel until it is read from the read end of the pipe.   For further details, see
       pipe(7).

       If flags is 0, then pipe2() is the same as pipe(). The following values can be bitwise ORed in flags to ob‐
       tain different behavior:

       O_CLOEXEC
              Set the close-on-exec (FD_CLOEXEC) flag on the two new file descriptors. See the description of the
              same flag in open(2) for reasons why this may be useful.

       O_DIRECT (since Linux 3.4)
              Create a pipe that performs I/O in "packet" mode. Each write(2) to the pipe is dealt with as a sepa‐
              rate packet, and read(2)s from the pipe will read one packet at a time. Note the following points:

              * Writes of greater than PIPE_BUF bytes (see pipe(7)) will be split into multiple packets.   The con‐
                 stant PIPE_BUF is defined in <limits.h>.

              * If a read(2) specifies a buffer size that is smaller than the next packet, then the requested number
                 of bytes are read, and the excess bytes in the packet are discarded. Specifying a buffer size of
                 PIPE_BUF will be sufficient to read the largest possible packets (see the previous point).

              * Zero-length packets are not supported. (A read(2) that specifies a buffer size of zero is a no-op,
                 and returns 0.)

              Older kernels that do not support this flag will indicate this via an EINVAL error.

              Since Linux 4.5, it is possible to change the O_DIRECT setting of a pipe file descriptor using fc‐
              ntl(2).

       O_NONBLOCK
              Set the O_NONBLOCK file status flag on the open file descriptions referred to by the new file descrip‐
              tors. Using this flag saves extra calls to fcntl(2) to achieve the same result.

RETURN VALUE
       On success, zero is returned. On error, -1 is returned, errno is set appropriately, and pipefd is left un‐
       changed.

       On Linux (and other systems), pipe() does not modify pipefd on failure. A requirement standardizing this be‐
       havior was added in POSIX.1-2016. The Linux-specific pipe2() system call likewise does not modify pipefd on
       failure.

ERRORS
       EFAULT pipefd is not valid.

       EINVAL (pipe2()) Invalid value in flags.

       EMFILE The per-process limit on the number of open file descriptors has been reached.

       ENFILE The system-wide limit on the total number of open files has been reached.

       ENFILE The user hard limit on memory that can be allocated for pipes has been reached and the caller is not
              privileged; see pipe(7).

VERSIONS
       pipe2() was added to Linux in version 2.6.27; glibc support is available starting with version 2.9.

NOTES
       The SystemV ABI on some architectures allows the use of more than one register for returning multiple values;
       several architectures (namely, Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64) (ab)use this feature in order to
       implement the pipe() system call in a functional manner: the call doesn't take any arguments and returns a
       pair of file descriptors as the return value on success. The glibc pipe() wrapper function transparently
       deals with this. See syscall(2) for information regarding registers used for storing second file descriptor.

CONFORMING TO
       pipe(): POSIX.1-2001, POSIX.1-2008.

       pipe2() is Linux-specific.

EXAMPLE
       The following program creates a pipe, and then fork(2)s to create a child process; the child inherits a dupli‐
       cate set of file descriptors that refer to the same pipe. After the fork(2), each process closes the file de‐
       scriptors that it doesn't need for the pipe (see pipe(7)). The parent then writes the string contained in the
       program's command-line argument to the pipe, and the child reads this string a byte at a time from the pipe
       and echoes it on standard output.

   Program source
       #include <sys/types.h>
       #include <sys/wait.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <string.h>

       int
       main(int argc, char *argv[])
       {
           int pipefd[2];
           pid_t cpid;
           char buf;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s <string>\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           if (pipe(pipefd) == -1) {
               perror("pipe");
               exit(EXIT_FAILURE);
           }

           cpid = fork();
           if (cpid == -1) {
               perror("fork");
               exit(EXIT_FAILURE);
           }

           if (cpid == 0) {    /* Child reads from pipe */
               close(pipefd[1]);          /* Close unused write end */

               while (read(pipefd[0], &buf, 1) > 0)
                   write(STDOUT_FILENO, &buf, 1);

               write(STDOUT_FILENO, "\n", 1);
               close(pipefd[0]);
               _exit(EXIT_SUCCESS);

           } else {            /* Parent writes argv[1] to pipe */
               close(pipefd[0]);          /* Close unused read end */
               write(pipefd[1], argv[1], strlen(argv[1]));
               close(pipefd[1]);          /* Reader will see EOF */
               wait(NULL);                /* Wait for child */
               exit(EXIT_SUCCESS);
           }
       }

SEE ALSO
       fork(2), read(2), socketpair(2), splice(2), tee(2), vmsplice(2), write(2), popen(3), pipe(7)

COLOPHON
       This page is part of release 5.05 of the Linux man-pages project. A description of the project, information
       about    reporting    bugs,    and    the    latest    version    of    this    page,    can   be   found   at
       https://www.kernel.org/doc/man-pages/.

Linux                                                 2019-03-06                                              PIPE(2)
